Lasing media in fiber format enjoys an advantage of high surface-to-volume ratio, resulting in efficient cooling, and consequent preference over other geometries for high power applications. Thus, fiber lasers rank prominently among the highest power lasers in existence today, with some producing beams of over ten kilowatts in power. Further, a fiber wave-guiding configuration can be structured to allow only a single mode of light output from the laser, resulting in superior beam quality relative to other geometries. Additional advantages of fiber waveguide laser geometry, insofar as the use of free-space optics is reduced or even eliminated, can include resistance to misalignment due to vibration or temperature fluctuations, ease of alignment, and compactness.
Having arrived at such high powers from the contemporary fiber laser material, i.e., silica, the ability to produce even higher powers has begun to become limited by the fundamental material properties of that lasing media. Thus, other materials possessing qualities superior to those of silica for purposes of fashioning fiber lasers are needed. Ceramic materials are strong candidates for this purpose. Several compounds are prominent among this class of materials as having unique potential to serve as useful high-power laser materials, for example, lutetium oxide (or Lutetia), yttria, and yttrium aluminum garnet (Y3Al5O12 or YAG). These materials are expected to be preferable to silica as fiber lasing media for two reasons related to their inherent material properties. First, the thermal conductivities of these materials are higher than those of silica. A higher thermal conductivity allows for waste heat to be extracted from the active lasing media more efficiently. Second, these materials typically permit higher levels of dopant to be introduced into their matrices than does silica. Higher dopant levels may result in achieving the same degree of absorption as in silica, while using shorter lengths of fiber relative to silica. This may be useful due to the fact that Stimulated Brilloiun Scattering, or SBS, a phenomenon which is deletrious to efficient high-power laser operation, is more prone to occur in longer lengths of fiber. SBS is a major concern to producers of higher power fiber lasers.
Presently, optical-quality YAG fiber has been created in single crystal form, only. However, ceramic fibers of these materials may be more desirable. The single crystal forms of these materials are more limited in their ability to incorporate high concentrations of certain dopants such as Neodymium, than the polycrystalline ceramic forms. Additionally, the methods by which single crystal fibers are produced—such as Laser Heated Pedestal Growth (LHPG), and Edge-Defined Film-Fed Growth (EDF, or EDFG)—generally cannot produce a fiber of diameter much less than 100 microns. This is due primarily to the fact that growth of single-crystals passes through a liquid phase, and when liquid phases of these materials are created with such small dimensions, capillary instabilities cause the liquid neck to collapse into a drop. However, in order to create fibers capable of delivering single mode beams, fibers with diameters on the order of 20 microns or less are desirable. Extrusion, may be one practical method to create ceramic fibers of such small diameters.
In any optical material, optical loss due to scatter must be minimized. In a ceramic optical material, scatter typically originates at the grain boundaries. Optical scattering at grain boundaries of a ceramic depends on three things: index isotropy, homogeneity or absence of additional phases, and porosity. If the optical indicatrix is nonspherical, scatter will generally occur at grain boundaries as the light moves from one domain to another and experiences a change in the index of refraction. Therefore, hexagonally-close-packed materials, such as Sapphire, are typically unattractive as optical ceramics. However, for materials of cubic symmetry, such as those mentioned earlier, the light sees the same index of refraction as it moves from one domain to another, and so no refractive scatter is produced. Any optical inhomogeneity present at the grain boundary, such as a pore, or a different phase, will cause scatter.
Using appropriate preparation for both oxide and non-oxide bulk polycrystalline laser materials, a ceramic optic can be made of sufficiently low scatter as to be useful as a laser optical component. Commercially available examples include ZnSe and YAG. While the size of the grains themselves are irrelevant to scatter, they may have implications for the physical strength of the material, with smaller grains typically resulting in ceramic parts of greater strength and larger grains producing weaker strengths. For purposes of fabricating a fiber, smaller grains may also result in a smoother surface on the fiber than larger grains. Insofar as light can scatter from index inhomogeneities on the waveguide surface, large grain sizes in ceramic waveguides will be likely to “indirectly” result in increased scatter, in the absence of a polishing technique for smoothing the waveguide surface.
However, preparation of a pore-free ceramic is not trivial. Due to the thermodynamics of atomic mass-transport, the grains of a ceramic will change in size and shape when the material is heated. Depending on the initial porosity of the ceramic, and in consideration of various other factors such as the surface tension of the material's liquid phases, the possible presence of eutectics, the ambient pressure, and the heating rate, a given heating regimen may cause pores to either grow and increase in size, or to shrink and perhaps even to disappear entirely. Which direction the material takes depends on the details of that material's thermodynamics, in relation to the particulars of the heating regimen employed. The technical term used to describe such a pore-closing heating regimen is “sintering”. During sintering, the pores are more likely to disappear if the initial pre-sintering porosity of the ceramic is lower. Pores are generally less likely to disappear if the pre-sinter porosity is high. The initial grain size may also be a factor in pore elimination, with smaller grains being more likely to result in pore elimination than larger grains for identical initial porosities.
Creating a ceramic part of low initial, pre-sinter porosity involves considerable optimization of material chemistry and initial grain size. The net result of those preparations is an object termed a green body, which is a ceramic part in the approximate shape of the desired final geometry. This green body may have appropriately low initial porosity, and may also contain the presence of binders, or chemical materials needed solely for the purpose of holding the initial grains together, in their “green”, pre-sintered, state.
Generally, heating for the sintering process is accomplished using a furnace. One of the factors which promotes the elimination of pores during a sinter is pressure of a gas or air around the part. In some cases, it has been found that sintering a part, while the part is “immersed” in a high-pressure gas, is beneficial for elimination of the pores. An explanation for this phenomenon is not so much that the pressure simply presses the grains closer together. Rather, the pressure provides a thermodynamic potential which motivates pore elimination. In order to harness this reality for the purpose of effective sintering, a Hot Isostatic Press, or HIP furnace may be utilized. A HIP is a furnace equipped with a high-pressure enclosure.
In other cases it has been found that sintering the ceramic in a vacuum may also be beneficial for pore elimination. An explanation for pore elimination in a vacuum is that if the ceramic part is in a vacuum, then the pores within the ceramic part should be empty of gas. If there were no vacuum, then the gasses in the pores must at some point dissolve into the solid in order for the pore to be eliminated. However, if a vacuum is present, then there is no gas or other matter that requires dissolution into the solid, so pore closure should occur more readily. In the event that vacuum sintering is found to be preferable, one would typically use a vacuum furnace.
However, high costs are associated with using such furnaces, including replacement costs of heating elements with finite life spans. Additionally, processing chambers sizes of these furnaces, which provide the high pressures or vacuums, limit the sizes of fibers that may be processed. What is needed, therefore, is a low cost system and method for generating ceramic fibers for laser or other applications without the limitations and challenges set out above and using processes that may be performed at atmospheric pressure.